- Haskell-Cafe - Haskell.org

open-union
by Ben Foppa 09 Jun '14

09 Jun '14

Hi cafe. On a use case whim, I made the open-union package ( https://github.com/RobotGymnast/open-union) copying the basic idea from extensible-effects:Data.OpenUnion1. I haven't uploaded to hackage yet, on the chance that there's already something like this around. Here's the basic functionality: {-# LANGUAGE TypeOperators #-} {-# LANGUAGE ScopedTypeVariables #-} import Data.OpenUnion type MyUnion = Union (Char :| Int :| [()] :| Void) showMyUnion :: MyUnion -> String showMyUnion = (\(c :: Char) -> "char: " ++ show c) @> (\(i :: Int) -> "int: " ++ show i) @> (\(l :: [()]) -> "list length: " ++ show (length l)) @> typesExhausted main :: IO () main = do putStrLn $ showMyUnion $ liftUnion (4 :: Int) putStrLn $ showMyUnion $ liftUnion 'a' putStrLn $ showMyUnion $ liftUnion [(), ()] If any of the (@>) cases is omitted, a compile-time error occurs. If you try to lift a bad value to the union, a compile-time error occurs. Any thoughts? Is there already something like this around?

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Using quickcheck with test-framework (collect doesn't work)
by Rafael Almeida 07 Jun '14

07 Jun '14

Hello, I'm trying to use quickcheck with test-framework. QuickCheck site says I can use a function called collect to see statistics of data generation. The function "collect" doesn't seem to print anything, though. This is the output I get after running cabal test: Running 1 test suites... Test suite reference: RUNNING... Test suite reference: PASS Test suite logged to: dist/test/100doors-0.1-reference.log 1 of 1 test suites (1 of 1 test cases) passed. This is what I see inside the log: Test suite reference: RUNNING... Main: flipped door is different state: [OK, passed 100 tests] flipDoors creates a different list of doors unless empty: [OK, passed 100 tests] Properties Total Passed 2 2 Failed 0 0 Total 2 2 Test suite reference: PASS Test suite logged to: dist/test/100doors-0.1-reference.log This is my test code (stripping out the boring parts): instance Arbitrary Door where arbitrary = elements [Opened, Closed] prop_flipped_door_is_different_state door = door /= flipDoor door prop_flipDoors_creates_a_different_list_of_doors_unless_empty n doors = collect n $ (not $ null doors) ==> doors /= (flipDoors n doors) []'s Rafael

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Using quickcheck to check for errors
by Rafael Almeida 07 Jun '14

07 Jun '14

Hello, Let's say we want to quickcheck the function !!. Passing a negative index to it is an error. However, how can you make a check that it indeed errors in such situation? prop_negative_index_generates_error list = forAll negativeIndex \x -> ... What should I write in ...? []'s Rafael

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Postdoctoral Research Position: Type Inference in Functional Programming
by Ekaterina Komendantskaya 07 Jun '14

07 Jun '14

Hello, I am looking for a postdoctoral researcher to work on developing new algorithms for type inference in functional languages. Position is based at the School of Computing, University of Dundee, Scotland. Closing date for applications is 20 June 2014; starting date is flexible. Salary scale: £27,837 to £36,661 per annum. More details are available here: https://docs.google.com/document/d/1UUuf-BllmQ925vLV6jtzKreDcZYEOsF96dtj_IK… Please re-forward to potential applicants. Cheers, Katya Ekaterina Komendantskaya Senior Lecturer, Head of PhD Studies Room 1.04, Queen Mother Building School of Computing, University of Dundee Scotland, DD14HN Tel: (+44) 01382384820

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ZuriHac 2014 - Updates
by Bas van Dijk 05 Jun '14

05 Jun '14

Dear (potential) ZuriHac 2014 attendees, I would like to make a few announcements regarding ZuriHac 2014, a Haskell hackathon taking place in Zurich from Friday 6 June 2014 to Sunday 8 June 2014. Besides hacking on Haskell projects with core members of the community you will hear talks by Simon Marlow and Edward Kmett. Note that this event is open to any experience level, from beginners to gurus. In fact, one of the goals is to bring beginners in contact with experts so that the former can get a quick start in the Haskell community and the latter more help with their projects. For more information see: http://www.haskell.org/haskellwiki/ZuriHac2014 Registration ------------ I'm happy and sad to inform you that we've almost reached capacity for ZuriHac 2014. Happy because it looks like this might be the biggest Haskell Hackathon ever. Sad because I soon have to start adding new registrations to a waiting list. So if you want to join us, and haven't registered already, then please do so soon at: http://bit.ly/ZuriHac2014. Also if you've already registered but know you won't be able to make it, please cancel by emailing me to make room for others. T-Shirt ------- Like last year, each attendee will get a free ZuriHac T-shirt. If you've already registered, please tell me which sizes and model you would like to have: Male: S, M, L, XL, XXL Female: S, M, L, XL, XXL Host ---- Erudify, the company which is hosting ZuriHac, recently changed its name to Better. Do note that we haven't changed location. See our new website on why we have rebranded: http://better.com/en/rebranding Regards, The ZuriHac 2014 organizers

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ANNOUNCE: music-suite 1.7
by Hans Höglund 05 Jun '14

05 Jun '14

I am pleased to announce the release of Music Suite v1.7. The Music Suite is a collection of Haskell libraries for composition, analysis and manipulation of music. It is generic enough to handle electronic music and non-Western music theory, as well as most aspects of common music notation. It can read and write many standard representations such as MIDI, MusicXML, Lilypond and ABC notation. Getting it Install the Suite from Hackage: cabal install music-suite Documentation and examples: http://music-suite.github.io The source code is available on Github: https://github.com/music-suite For bug reports, please use the relevant Github tracker: https://github.com/music-suite/music-score/issues For questions, feedback and general discussion, see music-suite-discuss(a)googlegroups.com http://groups.google.com/d/forum/music-suite-discuss Highlights Some highlights of this release include: Many new time containers, including more fine-grained versions of Score and Behavior Better interface for Score, Voice and Chord Better interface to musical aspects (i.e. parts, pitches, dynamics and articulations) using lenses and traversals Polymorphic update of musical aspects, so representation can change mid-traversal Now supports phrase-wise traversals of scores and voices New SuperCollider backend Restructuring of backend code in terms of a single class HasBackend Backend can now be parameterized on container type, rather than just note type Better notation of dynamics and articulation For the full notes, see: https://github.com/music-suite/music-docs/blob/master/releases/Notes.md Stability Please note that the Suite is quite usable, parts of it are still experimental, and we expect the API to change slightly with every release up to v2.0.0 (think of it as optimistic versioning). If you have any problems with upgrading from a previous version, please post to the group below. Contributing If you are interested in contributing to the Suite, please join the Github organization (see the link above). In addition to code, we appreciate contributions in the form of tutorials, examples or musical compositions. Hopefully we may soon have a showcase of works created with Music Suite, like Diagram's gallery. Sincerly, Hans - Hans Höglund Composer, conductor and developer hans [at] hanshoglund.se hanshoglund.com https://twitter.com/hanshogl https://soundcloud.com/hanshoglund http://github.com/hanshoglund

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Getting list of constructors of a Type
by Alejandro Serrano Mena 04 Jun '14

04 Jun '14

Dear Haskell-Café, As part of my Google Summer of Code, I need to get the list of data constructors of a certain data type on a specific scenario. In my case, I'm getting a value of type Type (by calling exprType). I was expecting to be able to call getInfo directly on that value of type Type, but it doesn't have a name nor the constructors of Type are public. Thus, I'm not able to get that information :( Thanks in advance, Alejandro

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PROPOSAL: Record field type inference
by John Meacham 04 Jun '14

04 Jun '14

This is also available as html at http://repetae.net/computer/jhc/record_inference.html Record Type Inference ===================== An extension to the named field mechanism that will greatly enhance the utility of them when combined with the existing `DisambiguateRecordFields`, `RecordPuns`, and `RecordWildCards`. The proposal is to allow the types of record fields to be inferred by the normal type inference engine. It would look like ~~~~ {.haskell} data Rec = Rec {fieldA,fieldB,fieldC} f Rec { .. } = Rec { .. } where fieldA = True fieldB = 4 ~~~~ This would infer the types `Bool`, `Int`, and `forall a . a` for the fields of the record constructor and `f :: Rec -> Rec` for f. For the purposes of type checking the fields are treated as monomorphic and not generalized but defaulted like normal after typechecking the module. Other than infering the types of the record fields, the records have the normal syntax. The extensions `RecordPuns`, `RecordWildCards` and `DisambiguateRecordFields` will be enabled when record field inference is enabled. Selector functions will not be created for infered records, as in, the names are field labels and not functions. This means they do not share a namespace with functions and do not conflict with each other. Multiple records may have the same field names in the same module. This means the following is fine. ~~~~ {.haskell} data Rec1 = Rec1 {input, withFoo, withoutFoo } data Rec2 = Rec2 {input, withBar, withoutBar } f Rec1 { .. } = case input of [] -> Rec1 { .. } (x:xs) -> if hasFoo x then Rec1 { withFoo = x:withFoo, .. } else Rec1 { withoutFoo = x:withoutFoo, .. } ~~~~ Possible extensions ------------------- ### as-pattern disambiguation In order to make the disambiguation of record fields more useful without relying on the type checker for disambiguation, We can declare that variables explicitly bound to a constsructor in a pattern match use that constructor to disambiguate fields for operations on the variable. This is a purely syntactic transformation that can happen before typechecking. It can be used as follows. ~~~~ {.haskell} -- use the input bound by a Rec1 to update the input bound by a Rec2 f r1@Rec1 { input } r2@Rec2 {} = case input of xs | any hasBar xs = f r1 { input = [] } r2 { input } ~~~~ ### Field label inference It is concievable that we may want to infer the fields themselves of a record, as in: ~~~~ {.haskell} -- infer that R has the field labels bob and susan data R = R { ..} f x@R {bob} = R {susan = bob} ~~~~ In order to implement this, a pass through the file will collect every field label that is used with an explicit R constructor and treat the record as if it were declared with those names as infered fields.

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Happstack: moving the uploaded file
by Nikita Karetnikov 03 Jun '14

03 Jun '14

I’m trying to create a trivial Happstack application based on the example from the “File Uploads” section of the Happstack book [1]. It should work as follows: 1. If there is no file called ‘foo’ on disk, show the upload form at any page. 2. If the file exists, show its contents at ‘localhost:8000/file’ and the upload form at any other page. The difficulty is that initially the uploaded file is placed to a temporary location, but since I want to read the file later, I have to move it somewhere. ‘UploadFile-working.hs’ uses two handlers for that, which doesn’t look right. I’d like the code to look like ‘UploadFile.hs’, but it doesn’t work properly. Could anyone explain why? [1] http://happstack.com/docs/crashcourse/index.html

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ANNOUNCE: jhc 0.8.2
by John Meacham 02 Jun '14

02 Jun '14

Lots of internal improvements in this one, so may be of more interest to developers http://repetae.net/computer/jhc/ - completely replaced the front end parser/lexer with a new more flexible one. My rewritten one is about 3,250 lines shorter. - The new one uses a general precedence parser for application as well as infix operators, allowing consistent handling of bang, irrefutable, and @ patterns and allows prefix and infix operators with a higher precedence than application to be defined. Incidentally, this allows for ~ and @ in expressions. (~) = negate (prefixr 11) gives us a nice strongly binding negative and @ (infixr 12) is perfect for type directed name resolution without further overloading the humble dot. they parse exactly the same as they do in patterns so no new magic is being added to the symbols other than allowing them in expressions. - Layout algorithm is fully in the lexer now, no lexer/parser interaction needed and useful transformations happen between the two simplifying the parser. - updated a lot of the external libraries to new versions - improvements to the build based on feedback, now gives much better error messages about what went wrong. - include an experimental utility to automatically download 3rd party packages from hackage and install them, it has some kinks, but even if it can't do it automatically, it provides a template for easily modifying the package to work. - removed the last of the cabal-cruft from the library build system. - fixed a lot of bugs relating to errors not being caught early by jhc so they produce a cryptic message from the compiler later, attach source location to more errors for better reporting. - ghc 7.8 compatibilty contributed by others. (thanks!) - infix type constructor extension supported - field punning extension now supported - new option -fno-sugar to turn off all desugaring that would introduce any hidden dependencies, all literals are unboxed under it. WYSIWYG. - unboxed kinds now more general, work in more places - record field names may overlap - more efficient binary representation using LEB128 - Data.String added with mild magic. -- John Meacham - http://notanumber.net/

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